This invention generally relates to the medical arts and particularly concerns an automatic method for determining blood pressure. The medical arts have long recognized the need to measure arterial blood pressure as an indication of the general health of a patient.
The "auscultatory" method of blood pressure determination practiced by most physicians requires an inflatable blood pressure cuff, a pressure gauge and a stethoscope. The cuff is inflated until all blood flow in a limb has been stopped. Then the pressure is slowly released, by means of a manually operated needle valve, while the operator listens for Korotkoff sounds with a stethoscope placed on the skin over the occluded artery. The first Korotkoff sound usually appears at the systolic pressure, when the blood just begins to spurt through the occluded artery. The Korotkoff sounds continue, usually one occuring with each heartbeat until the pressure in the cuff is equivalent to the diastolic pressure. At this time, free blood flow is restored, and no further Korotkoff sounds are heard. This procedure is time consuming and requires a skilled attendant to accurately measure blood pressure. For this reason, the prior art has developed a multitude of instrumentation for automatic or semiautomatic determination of blood pressure. Such instrumentation may be applied internally and externally to the patient. An example of an internal blood pressure monitor includes those devices wherein penetration of the skin by means of a needle or the like through the arterial wall is effected. The pressure of the arterial pulse thereby being directly measured via a suitable pressure transducer. Such internal techniques risk the discomfort and infection of the patient and are certainly unsuitable for routine blood pressure checks. Externally applied instrumentation normally consists of a blood pressure cuff which is to apply pressure to the limb of the patient so that the arterial flow will be restricted. A suitable transducer similar to a stethoscope used in manual operation is then used to distinguish Korotkoff sounds which occur when the cuff pressure is between systolic and diastolic pressures. Many of these instruments require cuff inflation and deflation by a skilled operator by means of a bulb with a check valve for inflation and a needle valve for deflation. Recommended deflation rates for most accurate blood pressure measurement is between two and three millimeters of Mercury per heartbeat. To attain these rates an operator must continually adjust the needle valve because of the change in cuff volume. This requires the operators complete attention during the bleed procedure. Typically, some automatic blood pressure machines will have a light or audible signal emitted with each Korotkoff sound, so by remembering at what point of pressure the sounds appeared and disappeared the blood pressure may be noted. More sophisticated equipment automatically inflates the cuff and deflates the cuff at a fairly consistent rate by means of a pressure pump and a variable orifice bleed valve. The disadvantage of this method is the high cost of the variable orifice valve and control circuitry.